1. Field of the Invention
The present invention relates to improvements of a disc and the manufacturing method of the disc, and it particularly relates to a disc which allows to be printed with a good quality label on its flat surface.
2. Description of the Related Art
Recently, a disc (optical disc) such as a CD (compact disc) or a CD-ROM (Read Only Memory) is popular among users and is widely used as recorded sound information or picture information because it is capable of recording a large amount of information and performing quick random access by using an optical pick-up.
FIG. 1 is a partially cutout view in perspective of a disc of a prior art.
Referring to FIG. 1, a disc 1B of the prior art generally comprises a disc substrate 2 made of transparent disc molding resin material such as polycarbonate or acrylic resin, a reflecting layer 3 made of aluminum formed on an information signal surface 2c of the disc substrate 2 by a sputtering or a vapor deposition method, and a protecting layer 4 of several .mu.m made of ultraviolet rays curing resin formed on the reflecting layer 3 by a spin coat method.
Further, the disc 1B is provided with a center through hole 5 at the center thereof for being mounted on a reproducing apparatus (not shown), and a ring groove 4b (2b 3b) concentrically formed with the center through hole 5 on the same surface of the information signal surface 2c. A numeral 2b designates a ring groove directly formed on the disc substrate which corresponds to the ring groove 4b, which is inevitably formed on a disc molding process as described hereafter. The ring groove 2b has an inside diameter of 35 mm, a width of 1.0 mm and a depth of 0.4 mm. A numeral 3b indicates a portion of the ring groove 2b covered with the reflecting layer 3, and 4b a portion of the ring groove 2b covered with the protecting layer 4 over the reflecting layer 3. Therefore, a top surface of the protecting layer 4 is exposed to the atmosphere and divided into two areas by the ring groove 4b, an inner flat area 4a which is used for clamping the disc 1B to a disc playing device, and an outer flat area 4c corresponding to the upper side of the information signal surface 2c. On the outer flat area 4c, a label 6B, a design and information of which are often related to the information signals to be recorded, is printed by a screen printing machine or an offset lithography machine (not shown).
As well known, the information signals are reproduced by irradiating a laser beam from an optical pick-up of the reproducing apparatus on the information surface 3c through an opposite surface 2e of the flat area 4c, wherein the numeral 3c indicates a part of the information signal surface 2c covered with the reflecting layer 3.
Next, the description is given to the manufacturing method of the disc 1B of the prior art and the reason why the ring groove 2b is inevitably formed on the disc substrate 2 on the disc molding process referring to FIG. 2.
FIG. 2 is a sectional view showing a disc metal mold for molding a disc substrate in a prior art.
Generally, the disc 1B is produced as a mass production process using a disc metal mold 10 having a fixed part and a movable part which is moved closer to the fixed part to form a cavity 22 for molding the disc substrate 2 as described hereafter referring to FIG. 2.
Referring to FIG. 2, 11 designates a fixed platform 11 of the fixed part for attaching a fixed core 13, 12 a spool bushing having a inlet 12a provided at the center of the fixed platform 11 for introducing disc mold resin into a cavity 22, a stamper 14 having an information signal surface 14c on which series of pits concentrically formed for forming inverted replicas of the pits on the disc 1B correspondingly with the information signals, and having a center hole 14b at a center thereof, 15 a retainer having a collar 15a protruding downward for supporting the stamper 14 at a center thereof, and 16 a flange having a collar 16a for supporting the stamper 14 at a periphery thereof. The stamper 14 is attached on the fixed core 13 provided on the fixed platform 11 by causing an outer peripheral portion and an inner peripheral portion of the center hole 14b thereof to engage with the collar 16a of the flange 16 and the collar 15a of the retainer 15, respectively.
On the other hand, a punch 18 for punching the center through hole 5 in the disc 1B is snug-fitted and is slidable in directions of arrows A1, A2 in a movable platform 17 of the movable part, and a top end of the punch 18 opposes a bottom of the spool bushing 12. A movable core 19 is also attached on the movable platform 17 by causing an outer peripheral part and an inner peripheral part thereof respectively to engage with an outer peripheral retainer 21 and an inner peripheral retainer 20 through which a part of the punch 18 is protruding into a cavity 22. Further, both top surfaces of the movable core 19 and the inner peripheral retainer 20 are polished to be kept flush each other, and the upper surface of the outer peripheral retainer 21 is made higher than both the top surfaces of the movable core 19 and the inner peripheral retainer 20 and contacts with a bottom of the collar 16a of the flange 16. It should be noted that the outer and inner peripheral retainers 21, 20 are provided in a detachable manner on the movable platform 17 so as to enable the top surface of the movable core 19 to be polished.
Accordingly, a space formed between the movable core 19 and the stamper 14 becomes the cavity 22 for molding the disc substrate 2. The disc substrate 2 is molded by injecting disc mold resin material from the inlet 12a into the cavity 22. At that time, each shape of the distal end surface of the spool bush 12, the protruding part of the collar 15a and the information signal surface 14c is respectively transferred as a replica on the disc substrate 2, i.e., the inner flat surface 2a, the ring groove 2b and the information signal surface 2c having the series of pits corresponding to the information signals as shown in FIG. 1. Then, the punch 18 is pushed in the direction of the arrow A1 in the disc metal mold 10 to make the center through hole 5, thus, the disc substrate 2 is molded.
After the disc substrate 2 taken out from the disc metal mold 10 is provided with the reflecting layer 3 on the information signal surface 2c, the inner flat surface 2a and the ring groove 2b, the protecting layer 4 is coated on the entire reflecting layer 3 and then the label 6B is printed on the outer flat area 4c to obtain the disc 1B.
As shown in FIG. 1, each of the inner flat surface 2a, the ring groove 2b and the information signal surface 2c covered by the reflecting layer 3 is respectively shown as a numeral 3a, 3b and 3c, and the same part on which the protecting layer 4 is formed is corresponding to the inner flat area 4a, the ring groove 4b and the outer flat area 4c.
As clearly understood from the above description, the ring groove 2b(3b,4b) is inevitably formed on the disc substrate 2 which divides a generally flat surface of the disc substrate 2 into two parts, i.e., the inner flat surface 2a and the information signal surface 2c as long as employing such a disc manufacturing method as attaching the stamper 14 by causing the inner periphery of the center hole 14b thereof to engage with the collar 15a of the retainer 15 and making the passing center hole 5 of the disc 1B in the disc metal mold 10.
In the disc 1B of the prior art as shown in FIG. 1, the label 6B is usually printed only on the outer flat surface 4c, however, there is a demand for printing the label 6B on the entire surface of the disc leaving the center through hole 5, which poses a problem that the ring groove 4b prevents from being printed with a label in good appearance because the ring groove 4b interrupts a part of the label 6b.
In another prior art, there is such a disc manufacturing method as follows:
A disc substrate is molded so as to have thoroughly a flat surface over the whole information signal surface by using a disc metal mold in which no center through hole forming device is provided. After the molded disc is taken out from the disc metal mold, a center through hole is punched out in the disc substrate at a center thereof by a mechanical method.
According to the above method, as the whole top surface of the molded disc except the center through hole is flat, the label is printable on the whole top surface of which the reflecting layer and a protecting layer is formed, however, it requires such a difficult process as manually positioning precisely the center through hole under a microscope upon punching thereof, which poses a problem i.e. it makes the mass production of such discs impractical due to an increase of manpower.
Therefore, there is a demand to provide a disc and a manufacturing method of the disc for satisfying both conditions, i.e., one is to enable mass production of the discs by using the disc metal mold with which the passing center hole is casted together as shown in FIG. 1 and another is to enable printing the label on the entire top surface leaving the center through hole on the disc.